The present invention has arisen in response to demand in the power generating industry for improved filtration of liquids containing suspended solids present in the steam condensate system in nuclear and fossil fuel electrical generation stations. The benchmark requirement is the removal of suspended solids from steam condensate streams, which accumulate over time at very low levels, on the order of 5 to 20 parts per billion (ppb), of which a percentage may be radioactive. These operations are known as "condensate polishing."
The removal of these contaminants from the condensate is essential to the safety, prevention of corrosion within the piping system and of damage to pumps, valves and other components, and to the regulatory compliance of nuclear generating stations. The requirements of condensate polishing stands as one of the most demanding filtration operations.
In practice, condensate polishing is sometimes performed as a multistage filtration operation, employing a powdered ion exchange precoat filter as a pre-filter to remove a substantial increment of the suspended and dissolved solids and particularly the largest particles, from the condensate stream, followed by a deep bed ion exchange filter to remove the remaining contaminants.
The demands on the system are acute. While the state of the art systems are highly effective, they produce substantial volumes of spent ion exchange resins and spent precoat filters and components, which may be radioactive, and which accumulate at an undesirable level.
One of the demands of the system is the limited capability to regenerate ion exchange resins containing contaminants having particle sizes greater than about 2 to 3 micrometers in diameter. It is this limitation which the pre-filter is intended to satisfy, protecting the deep bed ion exchange resin final filter from such components.
There is increasing interest in the nuclear power industry in the usage of absolute filters as replacements for the ion exchange precoat filters frequently employed in the pre-filter role at present. Absolute filters are those having a surface barrier to the passage of particles larger than some particular size, as contrasted with depth filters, which entrain and entrap particles within the depth of the filter medium. Depth filters are generally limited in their capacities as debris accumulates, because the debris cannot be removed and once the filter is fully loaded, it must be discarded and replaced. Absolute filters are more amenable to cleaning.
A major driving force behind the exploration of new filtration techniques in the nuclear power generating industry is the problem of handling, storing and shipping radioactive waste materials. Indeed, the problem has recently become even more acute, as radwaste materials may no longer be shipped at all, and must be stored on site for the foreseeable future and beyond. The volume of such radwastes is a particular difficulty, as the storage areas are limited and such materials continue to be produced and to accumulate. The industry is acutely in need of techniques which reduce the volume of materials to be stored for indefinite period of time.
Two major categories of filters have been suggested for use in the pre-filter role in condensate polishing operations. These include the hollow fiber membrane filters, such as those disclosed in U.S. Pat. Nos. 4,767,539, 4,793,932, 4,931,186, 4,935,143 and 5,024,762, cited above, and pleated filters of nonwoven webs or fabrics. These filters eliminate the volume of the ion exchange resins which may be produced as low level radwaste in usual condensate polishing operations. While the hollow fiber membrane filters are excellent performers, they are costly. Nonwoven fabrics have heretofore had quite limited performance, particularly in the effectiveness of backwashing techniques. The permeability of nonwovens generally shows a progressive decline over time, indicating the limited effectiveness of backwashing operations and the accumulation of debris within the fabric which is not effectively removed by the backwash and which limits the filtration efficiency and effective life span of such filters. Such nonwovens are intrinsically less effective as absolute filters than asymmetric membrane filters.
The normal backwash techniques employed are based on the flow of a liquid medium through the filter medium in a reverse direction, i.e., from downstream side to the upstream side of the filter, sometimes with the inclusion of a gas, normally air, to produce turbulence in the liquid stream. The usual liquid is an increment of the filtrate previously filtered, although other liquids may be employed as well.
While the present invention has been developed in the context of condensate polishing operations in nuclear power generating plants, there are a wide variety of filtration requirements in other contexts suited for the employment of the present invention. The present invention is effective in any filtration operation where the level of contaminant is particularly high, or where very large volumes having low levels of contaminant must be filtered, and where long filter life and effective cleaning are of particular significance.